Developmental exposure to noninherited maternal antigens induces CD4 ⁺ T regulatory cells: Relevance to mechanism of heart allograft tolerance

We hypothesize that developmental exposure to noninherited maternal Ags (NIMA) results in alloantigen-specific natural and adaptive T regulatory (T _R) cells. We compared offspring exposed to maternal H-2^d (NIMA^d) with nonexposed controls. In vitro assays did not reveal any differences in T cell responses pretransplant. Adoptive transfer assays revealed lower lymphoproliferation and greater cell surface TGF-β expression on CD4⁺ T cells of NIMA^d-exposed vs control splenocytes. NIMA^d-exposed splenocytes exhibited bystander suppression of tetanus-specific delayed-type hypersensitivity responses, which was reversed with Abs to TGF-β and IL-10. Allospecific T effector cells were induced in all mice upon i.v. challenge with B6D2F1 splenocytes or a DBA/2 heart transplant, but were controlled in NIMA^d-exposed mice by T _R cells to varying degrees. Some (40%) NIMA^d-exposed mice accepted a DBA/2 allograft while others (60%) rejected in delayed fashion. Rejector and acceptor NIMA^d-exposed mice had reduced T effector responses and increased Foxp3⁺ T_R cells (CD4 ⁺CD25⁺Foxp3⁺ T_R) in spleen and lymph nodes compared with controls. The key features distinguishing NIMA ^d-exposed acceptors from all other mice were: 1) higher frequency of IL-10- and TGF-β-producing cells primarily in the CD4⁺CD25 ⁺ T cell subset within lymph nodes and allografts, 2) a suppressed delayed-type hypersensitivity response to B6D2F1 Ags, and 3) allografts enriched in LAP⁺, Foxp3⁺, and CD4⁺ T cells, with few CD8⁺ T cells. We conclude that the beneficial NIMA effect is due to induction of NIMA-specific T_R cells during ontogeny. Their persistence in the adult, and the ability of the host to mobilize them to the graft, may determine whether NIMA-specific tolerance is achieved.